Hybridization, Hennig, and the Species-as-Individuals Hypothesis

Ian Smith

In a recent work (Book X), I argued that currently existing sexual species can flourish,
and that their flourishing is the basis of their intrinsic value. In that work, I
appealed to the Hennigian Species Concept (HSC) along with the Species-as-Individuals
(S-A-I) hypothesis in understanding what species are, for understanding what species
are will help us to understand how they could flourish. Under the HSC, species are
reproductively isolated populations or metapopulations. When they speciate, they create
two new daughter taxa, and this process thereby renders the parent species dissolved.
So, species persist until they dissolve or go extinct.

In that work, I mentioned the problem of hybridization, which is a great example of
how lineages can supposedly converge on one another. I dismissed the problem of hybridization
by arguing in the following way: Suppose that two sexual “species” hybridize such
that the organisms of such “species” sexually produce a new sexual species (whose
organisms are fertile) that happens to be reproductively isolated from the two parental
“species.” This would presumably be speciation not based on a split in lineages, but
based on a convergence among lineages to form a new species. In other words, what
would appear to occur here is reticulation in the phylogenetic species tree rather
than divergence, thus appearing to violate the logic of the phylogenetic species tree.
Those who write on this kind of speciation tend to assume that the two “species” are
truly of different species. But if we do not view the two parental populations as
of different species, but instead understand them to be of the same species, then
there is no reticulating problem. The correct tree, on the HSC, should have these
two populations in the same species lineage. Why is this the case? The two populations,
in hybridizing and producing organisms that are fertile, are not reproductively isolated.
They cannot logically be reproductively isolated if they have been able to hybridize
and produce organisms that are fertile.

I now realize that the conjunction of the HSC and the S-A-I hypothesis in light of
the hybridization problem leads to potential inconsistency. The populations mentioned
above that “hybridize” cannot easily be seen as part of the same individual, prior
to the “hybridization.” As David Hull argued, metapopulations that are individuals
are considered as such (partly) because they occasionally exchange individual organisms.
But populations that do not occasionally exchange individual organisms cannot be seen
as individuals. He uses an analogy with two drops of mercury to illustrate: as long
the two drops of mercury do not come together, they are not one drop. That they could
be viewed as potentially of the same drop of mercury does not of course mean that
they are at the time of being separate. Accordingly, it does not appear that the two
populations in the above passage can be considered as part of the same species prior
to the “hybridization,” assuming we accept the S-A-I hypothesis, since they are not
yet exchanging organisms.

The hybridization problem is not just a problem with the conjunction of the S-A-I
hypothesis and the HSC, however. The problem is deeper, for it affects the view that
there is a phylogenetic tree that correctly represents the evolutionary history of
speciation (or even whether there is more than one tree that could represent the evolutionary
histories of speciation). Phylogenetic systematists could just deny that hybridization
occurs by explaining it away, as I have attempted to do. But they may not be able
to do so very easily.

This project will thus consider if it is indeed the case that true hybridization is
inconsistent with phylogenetic systematics. It will also consider the narrower question
for my view, whether the conjunction of the HSC and the S-A-I hypothesis is tenable,
considering the hybridization problem.